1. Field of the Invention
A DC converter according to the present invention particularly relates to a DC converter which supports a first drive method using a PWM (Pulse Width Modulation) pulse signal and a second drive method using a PFM (Pulse Frequency Modulation) pulse signal.
2. Description of Related Art
In recent years, a DC converter has often been used as a power control circuit in an electronic circuit. A DC converter is a voltage converter which is supplied with a DC voltage as an input voltage and then outputs an output voltage having a different voltage value. As an example of a DC converter, there is a switching regulator generating an output voltage by driving an output transistor on the basis of a pulse signal. Switching regulators are widely used due to the high efficiency of power conversion.
An example of the switching regulator is disclosed in Patent Document 1. FIG. 8 shows a block diagram of a switching regulator 100 described in Patent Document 1. The switching regulator 100 generates a feedback voltage VFB by dividing an output voltage VOUT by means of resistors R111 and R112, comparing the feedback voltage VFB with a reference voltage VREF, and generates a one-shot pulse signal having a fixed off-time. A constant voltage source VOS, a reference circuit 111, a transconductance amplifier 112, a constant current source 113, a current comparator 114, and a one-shot circuit 116 are used to generate the one-shot pulse signal. The one-shot pulse signal is then supplied to a drive circuit 120 via an inverter 117, a NAND circuit 118, and an AND circuit 119. The drive circuit 120 drives a push-pull switch 130 on the basis of the one-shot pulse signal. The push-pull switch 130 generates the output voltage VOUT with use of an inductor L and a capacitor C.
At this point, pulse intervals are set for the one-shot pulse signal on the basis of a difference between the reference voltage VREF and the feedback voltage VFB. A method of driving the push-pull switch 130 on the basis of a signal changing the frequency of a pulse and having a fixed off-period (a fixed high-level period) as described above is called the PFM (Pulse Frequency Modulation) drive method.
In addition, the supply of the one-shot pulse signal to the drive circuit 120 is interrupted in the switching regulator 100 when the output voltage VOUT exceeds a specified voltage. In the example shown in FIG. 8, a hysteresis comparator 115 detects a fact that the output voltage VOUT reached a specified voltage, from the feedback voltage VFB, and puts the NAND circuit 118 and the AND circuit 119 in an interrupted state. Such a drive method using a pulse signal including the no-signal state is called a VFM (Variable Frequency Modulation) drive method.
In the switching regulator 100, the output voltage VOUT is generated by the PFM drive method, and when the voltage VOUT reaches a specified voltage value, a voltage is generated by the VFM drive method. At this point, the push-pull switch 130 is substantially in a stop state during a drive period by the VFM drive method. Such a drive period by the VFM drive method allows a reduction of power consumption in the switching regulator 100.    [Patent Document 1] Japanese Patent Application Publication No. Hei 6-303766